Air is an insulator. You put more air in the cyl, it makes it harder for the spark to jump the gap, thus a smaller gap is needed..

BTW: Love that 29 Fountain!!

NGK Spark Plugs USA has a tech support for just the questions you are asking : 1-877-473-6767
They are located at 46929 Magellan Drive,Wixom,MI 48393.Ask for Russell Senior he is a application engineer for the company.
I would also suggest that you look into Iridium or Platinum-type plugs - they are fine wire plugs and resist gap erosion and come factory set at the gap .They also do not suggest changing the gap set at the factory. If you are going to change the plugs make sure the plugs are torqued with a proper wrench and use dielectric grease on the terminal where the spark-plug boot connects to the plug and NEVER use a lubricant on the threads because it changes the torgue values......Good luck with those new engines.


Jim

I run ngk 9 in my blower engines. I believe you should run the coldest plug possible without getting fouling issues idling.

Thanks, It is a work in progress. I would love a 35 or 38; maybee in a few more years I will move up.

Thanks, I will give him a call.

All great info. I really appreciate everyone's responses.

Same here. Tapered seat plug. .460 Reach. Been running them since 2009 with no issues. Before that, autolite AR134. Switched to NGK since they are easier to find. Never fouled one, ever. Gapped at .045 with a CD ignition. Stickin with the NGK 5673.

FYI, mercury used the delco MR41T in their SC 600 and 525SC, and 800sc engines. Worked good for them too.

That looks like my old fountain! If it is, we had alot of fun in that boat. For some reason I cant see the picture any larger but the hatch looks familiar. We had twin 502s with 250B&M and ran 93mph back in 1999.

Scotty I bet it is. I will PM you.